The present invention relates to pneumatic conveying apparatus and in particular to a flap valve arrangement for use with a line charging device using a screw conveyor and discharge chamber for supplying solid particulate material to a pneumatic conveying line. The invention also relates to a valve for closing the outlet of a screw conveyor where the shaft of the screw impeller extends through the outlet.
Prior to the present invention it was generally known to provide pneumatic conveying and line charging devices for conveying solid particulate material including a screw impeller rotatably mounted within the bore of a casing for transporting the particulate material from a source such as a hopper to a casing outlet. A discharge chamber or wind box is flow connected to the casing outlet for receiving the particulate material. Gaseous fluid under pressure is supplied to the discharge chamber for entraining the particulate material and conveying such material out of the discharge chamber and through a pneumatic conveying line which is connected to the discharge chamber.
Early designs of pneumatic conveying apparatus of the type to which the present invention relates are generally known as shown in U.S. Pat. No. 1,677,119, issued July 10, 1928. This design employed a front and rear bearing to rotatably support the screw impeller at both ends. A compressing screw formed by either a reducing pitch screw or a reducing diameter casing bore or an increasing diameter screw impeller shaft or combination thereof serves to compact material within the casing bore to thereby form a seal of material in the casing between the material outlet of the casing and the material inlet of the casing. This material seal serves to prevent "blowback" from the discharge chamber into the feed material hopper, i.e. prevent air under pressure used for conveying material through the line from short-circuiting into the material feed hopper through the bore in the casing rather than conveying material through the pneumatic conveying line.
a later development in pneumatic conveying apparatus of the type to which the present invention relates is generally shown in U.S. Pat. No. 2,299,470, issued Oct. 20, 1942. In that apparatus, the front bearing supporting the screw impeller of the early design is eliminated. An overhung screw impeller is supported by a rigid rear bearing. The front end of the screw impeller is kept centered within the bore or pump barrel by employing a heavy impeller shaft and by keeping the barrel full of material. In this latter device, a weighted flap valve is used for closing the outlet of the casing. During initial start-up of the pump, material is conveyed by the screw impeller from the inlet toward the outlet of the pump barrel. The valve serves to assist in forming a material seal at the outlet by remaining closed until there is sufficient material build-up to form the seal and overcome the weight closing the valve. Once the material seal is formed, the flap valve will swing into the discharge chamber away from the casing outlet and remain open so long as material is being conveyed. The material seal prevents "blow back" . If material flow through the outlet stops, the valve swings closed to seal the end of the pump barrel.
U.S. Pat. No. 1,677,119 had the advantage over U.S. Pat. No. 2,299,470 in that the pump screw was supported by bearings at both ends which permitted the pump barrel to be operated at less than full clapacity and provided smooth operation. The later patent had the advantage of the flap valve aiding in the preventing of blowback.
Attempts have been made to combine the advantages of both pumps by employing a flap valve to aid in the formation of a seal of material at the outlet of the pump barrel and continue to utilize front and rear bearings to support the pump screw. Such apparatus are shown for example in U.S. Pat. No. 3,704,917 issued Dec. 5, 1972, U.S. Pat. No. 4,109,966 issued Aug. 29, 1978, U.S. Pat. No. 4,525,106, French Patent Specification No. 7039326 published June 2, 1972 and European Patent Specification No. 098340 filed Jan. 4, 1983.
In some of the prior art a side discharge arrangement has been utilized where the seal is formed perpendicular to the screw shaft. While this arrangement has been successfully used, many operators prefer the outlet of the pump screw to be aligned with the pump barrel as shown in U.S. Pat. No. 2,299,470 and the outlet of the discharge chamber to be in line with the pump barrel. Prior attempts to utilize an aligned outlet such as in U.S. Pat. No. 3,704,917 and European Patent Specification No. 098340 have had difficulties because of the movement of the valve element along the shaft and the wear created by the abrasive material being conveyed.
With swinging valves such as disclosed in European Patent Specification No. 098340, in order to allow the valve to swing freely relative to the shaft of the screw impeller, it is necessary to make the hole through the valve oversized. This leaves large gaps between the valve element and the screw shaft which are a source of leakage of conveying air and material. This annular member if mounted to the shaft will rotate and contact the valve and cause wear. If the annular member is supported by some means from the stationary outer barrel it creates a material obstruction which is undesirable and is energy consuming. In order to overcome this an oversized annular member is mounted on the shaft to provide a sealing point for the valve element. It would be desirable to eliminate this arrangement.
Other valves such as that shown in French Patent Specification No. 7039326 and U.S. Pat. No. 1,563,826 utilize a valve which employs a slot to permit the valve element to swing on the screw shaft. This slot prevents the valve from closing the outlet of the pump barrel.
It is desirable to provide a pneumatic conveying device of the screw impeller type which utilizes a flap valve capable of closing an axially aligned outlet of the pump barrel where the screw impeller is mounted in both front and rear bearings. This can be done according to the present invention by utilizing a novel valve arrangement capable of sealing at both the casing and substantially seal around the screw shaft.